The Determination of the Amplitude and Altitude of Stratospheric Warmings from Satellite-Measured Radiance Changes

Abstract

Satellite-measured radiances offer the first opportunity for investigating the total structure of winter stratospheric warmings, a phenomenon whose cause is still unknown despite intensive study since 1952, Vertical temperature profiles inferred from Nimbus III and IV SIRS 15 ? infrared measurements do not appear to be ideally suited for monitoring sudden warmings above the 10-mb level because of increased dependence, in the upper stratosphere, of inverse solutions of the radiative transfer equation on the climatological or ?first-guess? profiles employed. In this paper, a temperature model for stratospheric warmings is presented, from which radiance changes are calculated by direct solution of the integral radiative transfer equation. The model is based on a review of past warming events and its construction was guided by stated physical constraints. The radiance changes correspond to changes in the temperature structure from a pre-warming condition to an arbitrary phase of warming. Warmings by 20?90C centered at altitudes 20?45 km are considered. For a typical warm-layer thickness (20?25 km), it is shown that the central altitude and the amplitude of warming can be satisfactorily discriminated through the use of two simple parameters including the ratio of radiance change in SIRS channels 7 (?678 cm?1) and 8 (?669 cm?1). A nomogram is developed for specifying warming altitude and amplitude from observed radiance changes and is tested during a major warming which began in late December 1969 and which involved warming by more than 70C. Amplitudes and altitudes of warming were generally well specified, with rms departures from the observed values of 8C in amplitude and 2.4 km in altitude, if one unsuccessful specification for warming above 40 km is excluded. Possible explanations of this case are suggested.